NATURAL SCIENCES OF PHILADELPIIIA. 41 



off in November, and few are seen after this montli. I am not 

 sure that any remain all winter. 



Chroecocephalus pMladelphia. 



Extremely abundant, but only in spring and fall, being strictly 

 migrants here. The}- appear about the first week in April with 

 the last species, generally remain through most of May, and 

 return in September, remaining until the end of November. But 

 this general statement may be somewhat qualified in detail by 

 some observations that have interesting bearing, unless I wrongly 

 interpret them. In 18(59, from the beginning of April until the 

 22d, the birds were here in great numbers, and Avith a marked 

 preponderance of old full-plumaged individuals. Without au}^ 

 change in the weather, or other assignable cause, they suddenly 

 disappeared, presumably having moved northward. For a week 

 or ten da^'s scarcely one was to be seen ; when they became more 

 numerous, if possible, than before, and so continued through the 

 greater part of May. This last lot was almost entirely composed 

 of birds of the previous year, as shown by the plumage, there 

 being hai'dly a black-headed one among them. I should judge, 

 therefore, that the old birds migrate in advance of the young, and 

 also that they move the more hurriedly of the two. It is probable, 

 moreover, that only these adults were going to breed that 3'ear, 

 although it is pretty certain that some gulls, at least, reproduce 

 before gaining their perfect plumage. In the fall few black-headed 

 ones are ever seen, the hood being lost before they reach this 

 latitude ; but even at this season the old and young are readily 

 distinguished by other marks. Many return in September, and 

 then the earliest ones to come are the young. I verified the same 

 general observations through four migrations that I witnessed. 

 This year (1870) there were few — almost no — birds in part of 

 October, after the September young appeared to have passed on 

 but as I write (Nov. 18) the harbor is covered Avith thousands 

 of old ones in their winter dress. They appear at a little distance 

 almost pure white, and are beautifully conspicuous among the 

 other gulls and the terns with which i\xQy associate. 



Audubon is certainly mistaken in representing the female of 

 this species with a brown hood. The sexes are not distinguish- 

 able by any outward marks. If there is a United States gull with 

 the head of the color shown in the plate just alluded to, it must 



1871.] PARTI.— 4 



